GENEEAL DESCEIPTION OF THE MOON. Ill 



smooth clescendiiio- floors .such as we And in terrestrial vallej^s. The 

 rills are narrow crevices, often so narrow that their bottoms can not 

 be seen; they frequently branch, and in some instances are continued 

 as branching cracks for 100 miles or more. The characteristic rills ai"e 

 far more a))undant than the valleys, there being man}' scores already 

 described; the slighter are evidently the more numerous; a catalogue 

 of those visible in the best telescopes would probably amount to several 

 thousand. (See plates vi, ix, x.) 



It is a noteworthy fact that in the case of the rills, and in great 

 measure also in the valleys, the two sides of the fissure correspond so 

 that if brought together the rent would be closed. This indicates that 

 they are essentially cracks which have opened by their walls drawing 

 apart. Curiousl}' enough, as compared with rents in the earth's crust, 

 there is little trace of a change of level of the two sides of these rills — 

 only in one instance is there such a displacement well made out, that 

 known as the Straight Wall, where one side of the break is several hun- 

 dred feet above the other. (See plate ix.) 



In the region outside of the maria much of the general surface of 

 the moon between the numerous crater-like openings appears in the 

 best seeing with powerful telescopes to be beset with minute pits, often 

 so close together that their limits are so far confused that it appears 

 as honeycombed, or, rather, as a mass of furnace slag full of holes if 

 greatly magnified, through which the gases developed in melting the 

 mass escaped. (See plate v.) 



Perhaps the most exceptional feature of the lunar surface, as com- 

 pared with that of the earth, is found in the numerous systems of 

 radiating light bands, in all about thirty in number, which diverge 

 from patches of the same hue about certain of the crater-like pits. 

 These bands of light-colored material are generally narrow, not more 

 than a few miles in width; they extend for great distances, certain of 

 them being over 1,000 miles in length, one of them attaining- to 1,700 

 miles in linear extent. In one instance at least, in the crater named 

 Saussure, a ])and which intersects the pit may be seen crossing its 

 floor, and less distinctly, yet clearly enough, it appears on the steep 

 inside walls of the cavit}' . In no well-observed case do these radiating 

 streaks of light-colored material coincide with the before-mentioned 

 splits or rifts. Yet the assemblage of facts, though the observations 

 and the theories based upon them are very discrepant, lead us to 

 believe that the}- are in the nature of stains or sheets of matter on the 

 surface of the sphere, or perhaps in the mass of the crust. At some 

 points the rays of one system cross those of another in a manner that 

 indicates that the one is of later formation than the other. (See plates 



III, VII, YIII.) 



